The present invention generally relates to a line head driving apparatus, which is applied to, for example, a thermal printing device.
A line head driving apparatus is used in a thermal printing device, a display device or the like. A line head has a plurality of thermal elements, which are arranged into a line. The thermal elements are driven in accordance with image data so that dot images are formed on a recording sheet or a display device.
As is known, a thermal head is a line head. A conventional thermal head has a layer structure in which a ceramic layer, a glaze layer and a resistance layer are laminated in this order on a heat sinking plate. The resistance layer is electrically driven via an electrode so that it emits heat. An ink sheet located in the vicinity of the resistance layer is heated by the resistance layer being driven so that a dot is recorded on a recording sheet through the ink sheet. Alternatively, a heat sensitive sheet is directly heated by the resistance layer so that a dot is recorded thereon.
Heat generated by the heat element being driven is, on the other hand, accumulated in the ceramic layer between the glaze layer and the heat sinking plate. Each thermal element is affected by its peripheral thermal elements as well as a past profile which shows how each thermal element was driven in the past. Thus, the heat storage states of the thermal elements are different from each other. It will be noted that the density of dot varies in accordance with a variation in the heat storage state of each heat element.
There is known a line head driving apparatus which is directed to overcoming the above-mentioned problems (see Japanese Laid-Open Patent Application No. 60-131262). The line head driving apparatus disclosed in the Japanese application drives the heat elements arranged into a line, as shown in FIG. 1. Referring to FIG. 1, D.sub.0 indicates a dot of interest. The heat storage state of the thermal element corresponding to the dot D.sub.0 is determined on the basis of binary image data which are applied to the thermal elements which form peripheral dots D.sub.1 -D.sub.21. Dots D.sub.1 -D.sub.4 are located in the same line as the dot D.sub.0. The dots D.sub.9, D.sub.15, D.sub.20 and D.sub.21 lead the dot D.sub.0 by one to five lines. The dots D.sub.5 -D.sub.7, D.sub.9 -D.sub.11, D.sub.12 -D.sub.14 and D.sub.16 -D.sub.18 lead the dot D.sub.0 of interest by one or two lines. Binary data applied to the thermal elements related to the dots D.sub.1 -D.sub.21 are weighted in accordance with the positions thereof with respect to the dot D.sub.0. The heat storage state of the thermal element corresponding to the dot D.sub.O is calculated from weighted binary data. Binary data applied to the thermal element corresponding to the dot D.sub.0, that is, the amount of heat generated thereby, is compensated on the basis of the calculated heat storage state.
As described above, the heat storage state of the thermal element of interest is obtained from binary data related to the 21 peripheral dots D.sub.1 -D.sub.21. It will be noted that each thermal element is supplied with binary data, which indicates whether the corresponding dot is white or black. In this case, the procedure for calculating the heat storage state is not much complex. However, in a case where multilevel image data is applied to each thermal element, the heat storage state calculation procedure is very complex and a large amount of data must be processed in order to obtain the heat storage state of the thermal element of interest by referring to the image data related to the peripheral dots. Further, the line head driving apparatus disclosed in the aforementioned Japanese application encounters a problem illustrated in FIGS. 2 and 3. When a totally filled area 10c is printed on a sheet, the density of a front portion 10a thereof (which amounts to, for example, about 200 lines) increases gradually, and the density of an end portion thereof decreases gradually. This phenomenon is caused by the heat storage function of the thermal elements. As a result, the image quality of the totally filled area 10c is deteriorated.